Chiral metal complexes. 23. Stereoselective synthesis of α-aminoacidates

A series of ternary complexes of cobalt(III) containing a chiral linear N₄ tetradentate and either an optically active α-aminoacid or a substituted aminomalonic acid has been synthesised and characterised. A high degree of stereoselectivity is observed for these complexes. The crystal structure of one of these species, Λ-β₁-[Co(R,R-picchxn)(R-ABMA)]ClO₄·1.5H₂O, where R,R-picchxn is N,N′-di(2-picolyl) -1R,2R-diaminocyclohexane and ABMAH₂ is 2-amino-2-benzylpropanedicarboxylic acid, has been determined. Crystal data: C₂₈H₃₆N₅ClO_9.5Co is orthorhombic, space group P2₁2₁2₁, with a=12.659(4), b= 14.560(4), c=34.049(8) Å and Z=8: The structure was refined by block-matrix least-squares methods to R=0.071 for 3024 non-zero reflexions. The complex crystallises with two complex cations in the asymmetric unit. Each cation has a Λ-β₁ geometry, and the benzylaminomalonic acid ligand coordinates with an R (pro-S) absolute configuration. An intramolecular hydrogen bond between a tetradentate secondary amine group and the carboxyl group of the aminoacid is obvious in each cation, and this appears to confer thermodynamic stability on this diastereoisomer. Decarboxylation of the complexes containing coordinated aminomalonic acids gives corresponding diastereoisomers of aminoacids in high optical yield. The nature of the precursor and final complexes casts some light on the origins of the chiral discriminations important in these species and their reactivity.